1. Field of the Invention.
The present invention relates to a redundant common communication system in which a plurality of nodes or stations are coupled to a plurality of redundant communication channels (such as communication busses) for transmitting data packets among the various stations.
2. Description of the Prior Art.
Distributed data processing and process control systems generally use a common communication system, such as a communication bus, for transferring packets of data among the various stations. An example of this type of system is a distributed process control system, in which the stations include process controllers, control consoles, and other data processing devices which share a common communication bus.
Because of the need for high reliability, distributed process control systems generally utilize a multi-channel redundant common communication system for transmitting packets of data among the stations. This redundant system normally takes the form of a pimary communication bus and a redundant communication bus, both of which are coupled to each of the stations in the system. Each station is designed so that a failure of a single component should not disable both busses. Upon failure of the primary bus, it is switched off and the redundant bus is switched on to transmit and receive data packets. If the redundant bus is operable, the process control system performs normally.
In the past, distributed systems having a redundant bus architecture have relied partially or entirely on direct human intervention for some aspects of switch-over between the primary and the redundant bus. For example, in one type of system, switching from the primary bus to the redundant bus occurs automatically, but the stations must be manually reset to the primary bus after it has been repaired.
One problem with the prior art systems which use a primary bus and an inactive redundant bus is that the operability of the inactive redundant bus is not known until there is a failure of the primary bus. If the redundant bus also is defective, the entire system fails.
Another disadvantage of the prior art systems is that significant amounts of data may be lost before the primary bus (or the redundant bus) is determined to have failed. Because process control systems generally control chemical, manufacturing or other industrial processes, the reliability of the data communication among stations is of critical importance.